Project Summary Respiratory syncytial virus (RSV) infects 64 million people and causes substantial mortality annually worldwide. There is no licensed RSV vaccine. Clinical trials of alum-adjuvanted, formalin-inactivated whole RSV (FI-RSV) vaccine caused vaccine-enhanced respiratory disease during epidemic season, which resulted in hospitalizations and two deaths. It is essential that an effective and safe RSV vaccine not induce vaccine-enhanced disease (VED) upon the immunized person?s exposure to live RSV. Currently licensed adjuvants cannot prevent or reverse RSV VED in animals. Our studies demonstrate that virus-like particle (VLP) vaccines presenting RSV F proteins (F VLP) confer protection against RSV by controlling lung viral replication without causing detectable VED. In addition, our preliminary data reveals that split RSV is safer than inactivated whole FI-RSV, and that certain Toll-like receptor (TLR) agonist combination adjuvants further improve split RSV vaccine efficacy and safety by preventing any residual VED response. During this STTR phase 1 project, we will assess the efficacy and safety of two promising novel RSV vaccine candidates in mouse and cotton rat preclinical animal models. The first candidate is an RSV F VLP vaccine formulated as a standalone agent or in combination with split RSV vaccine. We hypothesize that it may be possible to boost efficacy by combining these two RSV immunogens without compromising safety. Our second candidate is a split RSV vaccine administered with VED-preventing TLR agonist adjuvants. We have established rigorous methods for evaluating RSV vaccine safety, efficacy, and monitoring detailed innate and adaptive immunological responses in animal models. In Aim 1 of this project, we will focus on testing our newly developed F VLP and split RSV vaccines standalone or in combinations for efficacy and safety in mouse models. We will also determine whether split RSV vaccine in combination with low dose TLR agonists will improve the efficacy of RSV vaccines while preventing VED in mice. The goal of aim 2 is to investigate the immunogenicity, efficacy, and pulmonary disease of new RSV vaccine combinations (Split RSV +/- F VLP) in cotton rats after vaccination and challenge. We shall undertake additional studies of vaccine candidates with the most attractive efficacy and safety profiles in aged animal models during a Phase 2 STTR project.